Introduction: We have reported a syndrome of hypoketotic hypoglycaemia due to the de novo p.Glu17Lys mutation in AKT2, a serine threonine kinase central to insulin signalling. We aimed to evaluate further the physiological and cellular consequences of the p.Glu17Lys mutation.
Results: Two previously reported 17-year-old men heterozygous for the AKT2 p.Glu17Lys mutation, each of whom had experienced severe fasting hypoglycaemia in early life, were re-evaluated. Both had 37% body fat and normal levels of hepatic triglyceride and fasting de novo lipogenesis. In patient one, who has a family history of obesity-related insulin resistance (IR), blood glucose remained normal overnight with concomitantly low plasma insulin levels, and a normal increase in free fatty acids (FFA). In patient two hypoglycaemia with undetectable insulin occurred after 2 h, and recurred after oral glucose. The FFA profile in this patient was nearly flat. Oral glucose tolerance testing showed no evidence of diabetes in either patient. In primary fibroblasts from patient one low level basal phosphorylation of AKT and its substrates p70 S6 kinase and GSK3α/β was seen, but, surprisingly, their peak phosphorylation after insulin or EGF stimulation was attenuated. In 3T3-L1 cells conditionally overexpressing mutant but not wildtype AKT2 nuclear exclusion of FoxO1a was seen, but no difference in phosphorylation of AKT2 nor in basal or insulin-stimulated deoxyglucose uptake was observed.
Conclusion: The AKT2 p.Glu17Lys mutation produces mild constitutive activation of signalling in primary fibroblasts. In the 3T3-L1 model this is sufficient to suppress FoxO1a activity but not to activate glucose transport, consistent with the low glucose required to maintain normoglycaemia. The clinical importance of the attenuated peak cellular response to ligand is unclear, while the autocorrection of one patient with a family history of obesity-related IR may offer insights into the level at which insulin action is impaired in prevalent disease.
Declaration of funding: This work was supported by the Wellcome Trust, The Cambridge Gates Trust, NIHR Cambridge BRC, and MRC-CORD.